Sulfur-containing polymers are promising materials for optical applications. When the sulfur content is sufficiently high, polymers with a high index of refraction and low light-dispersion are obtained.
Esters of thio(meth)acrylic acid are particularly important as component units of polymers because they have a relatively high sulfur content. The poly(meth)acrylic acid esters of polythiols as described in EP-273,661 and EP-394,495, are suitable monomers for producing optical materials (see EP-273,710). Optical applications require high uniformity and purity of the starting materials. Heretofore, the preferred method of removing impurities and by-products has been to distill the monomers, which leads to a decreased yield due to partial polymerization, decomposition, etc.; this is particularly true in the case of high boiling compounds.
Because of the risk of thermally induced polymerization, as a result of the high boiling point, it has been found to be impracticable to purify higher polythio(meth) acrylates by distillation.
(Meth)acrylic acid esters of thioether alcohols have also been described (e.g. DE-A 38 38 350). However, neither these nor the vinyl compounds (claimed in EP-284,374) produced by Michael addition of multifunctional mercaptans to multiply unsaturated compounds provides a sufficient sulfur content. As a result, the index of refraction is too low, even though the optical dispersion is low, as desired.
Accordingly, there is a persistent need for transparent plastics with a high index of refraction, preferably plastics which are readily available or easy to produce industrially.
According to the method described hereinbelow, which comprises the principal claimed matter of the contemporaneous DE-P 42 34 257.0, starting with (meth)acrylic acid anhydride one can produce thio(meth)acrylic acid esters having sufficient purity for all customary applications. In the method, the (meth)acrylic acid anhydride, preferably dissolved in an inert water-immiscible solvent, is reacted with an optionally substituted C.sub.1-24 alkyl- or C.sub.6-24 arylthiol atoms, preferably in an aqueous alkaline medium. When the described method is employed with polythiols, a thioether polythio(meth)acrylate is produced as a byproduct in substantial proportions. Such a byproduct is also produced when (meth)acrylic acid chloride is reacted with polythiols.
The typical practice of one skilled in the art in comparable cases, is to separate the reaction products, the undesired byproducts (in this case the thioether polythio(meth)acrylates), and the auxiliary agents being tried; such separations may be, e.g., by means of distillation or with the aid of chromatography. However, as one could readily predict, industrial implementation of these techniques is impeded by the fact that distillation to separate these compounds is complex, costly and yield-lowering, and chromatography is complex and costly.
It has been discovered that, surprisingly, the polymer produced from the mixture of the unrefined products of the manufacturing process of synthesizing the thio(meth)acrylic acid ester has better characteristics for some applications than do the corresponding polymers from the purified monomers. Thus, poly(ethanedithiol dimethacrylate) has much poorer mechanical properties than the copolymer produced from the unrefined monomer mixture which is generated in the manufacturing process.